Strain measuring system



Feb. 5, 1946. B. F. LANGERA Em 2,394,019

STRAIN MEASURING SYSTEM Filed may 5. 1944 WITNESSES: INVENTOR5Berflard/F'Yanper and Kenna/h l. Nammacx.

ATTOR N EY Patented Feb. 5, 1946 UNITED STATES PATENT OFFICE STRAINMEASURING SYSTEM Bernard F. Langer, Squirrel Hill, and Kenneth L.

Wommack, Wilkinsburg, Pa., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of PennsylvaniaApplication May 5, 1944, Serial No. 534,222

- shall function simply and eflicientl and have a 8 Claims.

The present invention relates, generally, to systems for continuouslyindicating or measuring the deflection or strain of a member subject toa application, Serial No.'466,147, filed November 19,

1942, entitled Strain measuring system, now U. S. Patent No. 2,354,129,granted July 18, 1944, to a copending application of B. F. Langer and T.E. Browne, Jr., Serial No. 466,149, filed November 19, 1942, entitled"Strain measuring system,"

now U. S. Patent No. 2,354,130 granted 'July 18, 1944, and to another ofB. F. Langers copending applications, Serial No. 534,223 flied on thesame date as this application.

The present invention embodies the fundamental principles disclosed inthe above-mentioned copending applications and 'diflers therefrom in thematter of circuit details which measurably improve the performance ofthe strain measuring system when operating under adverse conditions. 1

The deflection or strain measuring system as hereinafter described andillustrated in the drawing is specifically adapted for measuring thetorsional deflections or twist of a shaft subject to,

torque, for the purpose of indicating the torque being transmitted bythe shaft. It is not to be construed, however, that the invention islimited to this application alone, since other forms of strain gaugesmay be substituted for that 11- lustrated to measure the deflection orstrain of a member subject either to tension or compression loading.Foran example, reference may be had to the strain gauge illustrated inPatent 2,231,702, S. L. Burgwin et a1. It will be apparent, however, toone skilled in the art that the specific invention illustrated in thedrawing for measuring the torque of a shaft provides accurate torqueindication in a degree hitherto unobtainable.

- A principal object of the present invention is to provide a system formeasuring thedeflection or strain of a member subject to a stress whichminimum number of parts.

Another object of the present invention is to provide a deflectionmeasuring system of the character referred to which shall compensate forvoltage fluctuations of the source of applied potential.

Another object of the present invention is to provide a deflection orstrain measuring system of the character referred to which shall beinsensitive to changes in ambient temperature.

Another object of the present invention is to provide a strain gaugecontrol circuit in which a substantially linear relationship between thequantity or voltage to be measured and the current in the indicatinginstrument coils is obtained..

A further object of this invention is to provide a strain measuringsystem which is calibrated for zero strain. That is, one in which the Icalibration line passes through zero for zero strain.

A still further object of this invention is to provide a strainmeasuring system of the character described in which the electricallosses are equalized among the several circuits.

Other objects and advantages will become more apparent from a study ofthe following specification when considered in conjunction with theaccompanying drawing, in which:

Figure 1 illustrates a'torque measuring system embodying the principlesof this invention; and

Fig. 2 is a sectional view taken on the line 11-11 of the strain gaugeof Fig. 1.

The specific strain gauge or torque measuring device schematicallyillustrated in the drawing is described in' detail in a copendingapplication of B. F. Langer and F. W. Godsey, Jr., Serial No..

458,378, filed September 15, 1942, entitled "Torque measuringdevicesforshafts and only such description which is necessary for acomplete un derstanding of the construction and operation of the straingauge will be included in this speciflcation.

Referring now to Figs. 1 and 2 of the drawing, numeral I denotes a shaftthe torque of which is to be measured; numeral 3 denotes a rotorassembly secured to the shaft and which in effect forms the armaturemember of the strain gauge;

and numeral 5 denotes an annular stationary member which isconcentrically positioned about the rotor assembly 3 and which in effectforms the core assembly of the strain gauge.

The'rotor assembly 3 comprises a pair of axially spaced torque rings 1and 9 secured to the shaft magnetic material. Centrally disposed betweenthe torque rings I and 3 is a third ring ll termed ,a reference ringlikewise supported and secured to the shaft on a bushing (not shown) ofnon-' magnetic material. The three rings thus secured to the shaft areso axially spaced and supported that a known gauge-length of shaft isincluded therebetween. The reference ring II has secured thereto aplurality of axially extending fingers i3 which'extend axially on eachside of the ring in the same axial plane. I and 3 has a plurality ofaxially extending fin gers l5 and II which are .of sufllcient length, tooverlap the extremities of the fingers i3 associated with reference ringII. The confronting faces of the co-operating fingers of the three ringsare so positioned during assembly that a small ai gap is includedtherebetween and the abovedescribed assembly is such that, for example,if torque were assumed to be transmitted from left to right of-the shaftin a clockwise direction, torsional deflections of the shaft between thetorque ring I and the reference ring ,ll would vso displace the'flngersl5 relative to the fingers l3 cooperating therewith that the air gap 13formed therebetween would be decreased and the torsional deflections ofthe shaft between reference ring ii and torque ring 3 would so displacethe fingers I'I relative to the fingers l3 co-operating therewith thatthe air gap 2| formed therebetween would beincreased.

The stationary member 5 comprises an outer annular shell 23 having threering'elements 25, 21, and 23 axially spaced and secured therewithin,each of which is positioned to be included in a transverse plane definedby one of the shaft ring elements. The inner bores of the ring elementsare of such diameter that small annular air gaps 3|, 33, and 35 areformed between the peripheries of the concentrically positionedconfronting faces. Included within the two annular recesses formedbetween the axially spaced rings 25, 21, and 23 are a pair of annularcoils 31 and 33 which are each connected as one leg in a conventionalbridge circuit. The other two legs of the bridge circuit are. formed bytheinductor 4|, a portion of which is shunted by a resistor 42. Bridgebalance may be obtained or approached by adjusting the slider 43 alongthe resistor.

; Upon suitable energization of the coils from the source of alternatingcurrent connected across the input terminals of the bridge circuit andindi-v cated generally by the sinusoidal wave, a flow of alternatingmagnetic flux may be induced to flow in the stationary member 5 and therotor 8888111! bly 3 in the instantaneous directions indicated by thearrows and for zero torque of the shaft the bridge circuit may beadjusted to a balanced condition, assuming perfect matching of the coilsand the magnetic circuits of the strain gauge, by movement of'theresistor slider 43 across the resistor 42. It may now, therefore, beseen that upon torsional deflection of the shaft due to clockwise torquefrom left to right. of the shaft as previouslydescribed, the air gaps iswill be decreased while the airgaps'fl will'be increased tosubstantially proportionally change'the values or the 70 alternatingmagnetic fluxes associated with each of the coils. Thus a voltage in onecoil will be increase while a voltagein the other coil will be decre tocause a voltage diflerence to appear betw the coils. This'voltagcdifference Each of thetorque rings f 5 tions'than at higher voltages.

functions in a manner similar to that of theis therefore measurableacross the output terminals of the bridge circuit.

The indicating instrument 45 is preferably of the cross-coiltype and isillustrated diagrammatically. It comprises a stationary permanent magnet41 and a movable'armature positioned to be influenced by the field ofthe magnet. The armature of the instrument includes a pair of coils 43and 5! mounted at an angle with respect to each other; the arrangementbeing such that both coils control the movements of the pointer toprovide an indication'corresponding to the ratio of the effects producedby the currents in the coils. Interconnecting the coil 43 of theindicating instrument with the output terminals of the bridge circuit isa circuit comprising abridge rectifier 53 having its input terminalsconnected across the inductor element 53 which in turn has a tappedportion thereof connected in series with the coil 43 and its negativeterminal is connected to the other side of coil 43. A series resistor 52is connected-on the alternating-current side of the rectifier 53, forthe purpose of providing a high circuit resistance which is preferablyconsiderably higher than that of the rectifier network or bridge. Thesecond coil iii of the indicating instrument is connected to the sourceof alternating potential by a circuit similar to that for the coil 43including a bridge rectifier 55 having its input terminals across thesecondary of a partially saturated transformer 32, the'primary of whichis in series circuit in one of the supply conductors connected to thesource of alternating potential. its negative terminal connected througha resistor 55 to one side of the coil 5! and its positive terminalconnected to the other 4 side of the coil 5|. A series resistor 51 isprovided in the circuit connecting the bridge rectifier to the secondarywinding of the partially saturated transformer and like the resistor 52this resistor is preferably of a much higher resistance value than therectifier network or bridge 55.

Series resistor 54 is provided for the purpose of increasing the load onthe rectifier 53 thereby increasing the voltage across the rectifier toa point where the rectifier operation-is more stable. For

example, without a resistance load in the instrument coil circuit therectifier output voltage may be only about three-tenths of one volt, atthis low voltage it has been found that the voltage is subject toconsiderably larger percentage fluctua- The resistor 56 measuringinstrument in opposition to that supplied by the rectifier 53. This isnecessary since in some installations it is not possible to completelybalance the bridge circuit by simple ad- Justment of the resistor slider43. i

It will be noted upon an inspection of Fig. 1 that both'coils 43 and 5l'of the-indicating instrument 45 are subject to voltage applicationsfrom the same source of alternating potential. Since, as previouslymentioned, the instrument provides an indication corresponding to theratio of the effects produced by the currents circulating in the coils,variations in the source of alternating potential, providing theelectrical losses in the two circuit branches are equal, will equallyaffect 25 both coils of the instrument due to the circuit arsaid firstcircuit.

of the pointer or the instrument II is, therefore,

' in view of the compensation for the variations 'be interpreted in. alimited sense. The only limitations are to be determined from the scopeor the appended claims.

We claim as our invention:

1. In a system for indicating a mechanical displacement, the combinationof, an electrical bridge circuit, means for I supplying alternatingcurrent tothe bridge circuit, means forming a part of the bridge circuitandresponsive to a mechanical displacement for producing an electri aichange in the bridge circuit, an electrical measu ing instrument havinga first and a second internal circuit providing an indicationcorresponding 1 9,394,070 I v I internalcircuit providing an indicationcomsponding to the ratio of the effects produced by the currents in saidinternal circuitsra first circuit including a rectifier networkinterconnecting the first internal circuit of said instrument and,

the electrical bridge circuit so that said first intemal circuit of saidinstrument has a voltage ap-.

plied thereacross corresponding to an electrical change of said bridgecircuit, means for matching the impedanceaof said electricalbridgecircuit and said first circuit, a second circuit including arectifier network electrically connected to said second internal circuitof said instrument, and a partially saturated transformerinterconnecting said second circuit and said means for supplyingaitemating current.

4. In a system for indicating a mechanical dis-1 placement, thecombination of, an. electrical bridge circuit, means for supplyingalternating current to the bridge circuit, means forming a part of thebridge circuit responsive to a mechanical displacement for producing anelectrical to the ratio of the effects produced by the currents in saidinternal circuits, a first circuit including a rectifier networkinterconnecting the first internal circuit of said instrument and theelectrical bridge circuit so that said first internal circuit of saidinstrument has a voltage applied thereacross corresponding to anelectrical change of said bridge circuit, a second circuit including arectifier, network electrically connected to said second internalcircuit of said instrument, and a partially saturated transformerinterconnecting said second circuit and said meansrfor supplyingalternating current, for introducing electrical losses in said secondcircuit similar to those of 2. In a system for placement, thecombination of. an electrical bridge circuit, means for supplyingalternating current to the bridge circuit, mean formin'ga part of thebridge circuit and responsive to a mechange in the bridge circuit, anelectrical measuring instrument having a first and a second intemalcircuit providing an indication corresponding to the ratio of theeffects produced by the currents in said internal circuits, a firstcircuit including a rectifier network electrically connected to thefirst internal circuit of said instrument, an element having inductanceproperties, said element having tapped portions thereofconnected acrosssaid bridge circuit to be energized according to the electrical changethereof and tapped indicating a mechanical dis- I portions connectedacross said first circuit, for

matching the impedances oi! the first circuit and the bridge circuit: asecond circuit including a rectifier network electrically connected tothe second internal circuit of said instrument, and a partiallysaturated transformer connecting said second circuit to said means forsupplying alter-' nating current.

5. In a system for indicating a mechanical displacement. the combinationof. an electrical bridge circuit, means for supplying alternatingchanical displacement for producing an electrical change in the bridgecircuit, an electrical measuring instrument having a first and a secondinternal circuit providing an indication corresponding to the ratio ofthe effects produced by the currents in said internal circuits, a firstcircuit including a rectifier network interconnecting the first internalcircuit of said instrument and the electrical bridge circuit so thatsaid first internal circuit of said instrument has a voltage appliedthereacrosscorresponding to an electrical change of said bridge circuit,a second circuit including a rectifier network electrically connected tosaid second internal circuit of said instrument, a partially saturatedtransformer interconnecting saidsecond circuit and said means forsupplying alternating current, and circuit means interconnecting saidfirst and second circuits for'providing a biasing voltage on said firstinternal circuit of said instrument. a

3. In a system ior indicating a mechanical dis? placement, thecombination of, an electrical bridge circuit, means for supplyingalternating current to the ,bridge circuit, means forming a part of thebridge circuit and responsive to a mechanical displacement for producingan electrical change in the bridge circuit, an electrical measuringinstrument having a first and a second current to the bridge circuit,means forming? part of the bridge'circuit responsive to a mech caldisplacement. for producing an electrical change in the bridge circuit,an electrical easuring instrument having a first and a seco d internalcircuit providing an indication corresponding to the ratio of theeffects produced by the currents in said internal circuits, a firstcircuit including a rectifier network electrically con-' nected to thefirst internal circuit of said instrument an element having inductanceproperties, said element having tapped portions thereof connected acrosssaid bridge circuit to be energized according to the electrical changethereof and tapped portions connected across said first circuit,

for matching the impedances of thefirst circuit and the bridge circuit;a second circuit including a rectifier network electrically connected tothe second internal circuit of said instrument, a partially saturatedtransformer connecting said second circuit to said means for supplyingalternating current, and circuit means interconnecting said first andsecond circuits for providing a biasing voltage on said first internalcircuit of saidinstrunient.

6. In a system for indicating a mechanical dis-' placement, thecombination -'of, an electrical bridge circuit, means for supplyingalternating current to the bridge circuit. means forming a asaao'zeacross said bridge circuit to be energized according to the electricalchange thereof and tapped portions connected across said first circuitfor matching the impedances of the first circuit and the bridge circuit;a second circuit including a rectifier network electrically connected tothe sec-- part of the bridge circuit responsive to a mechanicaldisplacement for producing anelectrical change in the bridge circuit, anelectrical measuring instrument having a first and a second internalcircuit providing an indication corresponding to the ratio ofthe-efiects produced by the currents in said internal circuits, a firstcircuit including a rectifier network electrically connected to thefirst internal circuit of said instrument an element having inductanceproperties, said element having tapped portions thereof connected acrosssaid bridge circuit to be energized accordingto the electrical changethereof and tapped portions connected across said first circuit, formatching the impedances of the first circuit and the bridge circuit; asecond circuit including 8. rectifier network electrically connected tothe sec- 0nd internal circuit of said instrument, a partially saturatedtransformer connecting said second circuit to said means for supplyingalternating cur rent, a resistor connected in series in said secondcircuit, circuit means connecting the first internal circuit of saidinstrument across said resistor, and

means in said first and second circuits for producing high electricalresistances therein whereby the resistances of said rectifier networksrepresent a small percentage of the total resistance of each of saidfirst and second circuits.

8. In a system for indicating a mechanical displacement, the combinationof, an electrical bridge circuit, means for supplying alternatingcurrent to the bridge circuit, means forming a part of the bridgecircuit and responsive to a mechanical displacement for producing anelectrical change in the bridge circuit, an electrical measuringinstrument having a first and a second internal circuit providing anindication corresponding to the ratio of the effects producedby thecurrents in said internal circuits, a first circuit including arectifier network interconnecting the first internal circuit of saidinstrument and the electrical bridge circuit so that said first internalcircuit of said instrument has a voltage applied thereacrosscorresponding to an electrical change of said'bridge circuit, a secondcircuit including a rectifier network electrically connected to saidsecondinternal circuit of said instrument, a partially saturatedtransformer interconnecting said second circuit and said means forsupplying alter hating current, circuit meansinterccnnecting the firstand second circuits for providing a biasing

